Large multiple processor computing systems often employ a directory cache that caches or stores a copy of information contained in a master directory. The master directory typically contains information regarding copies of memory lines that are stored in the system's various caches. For example, the information stored in a master directory (and thus the directory cache) may comprise information indicating 1) where copies of memory lines are stored, 2) whether the memory lines are stored in single or multiple locations, and 3) whether a memory line has been reserved for exclusive access by a particular cache (meaning that the memory line may be subject to modification).
A directory cache can be useful in that it reduces the number of main memory accesses (and particularly writes) that are undertaken solely for the purpose of keeping the master directory up-to-date (i.e., it reduces memory accesses that do not result in a memory line actually being updated). This can yield both increased effective memory bandwidth and reduced memory power consumption.